1. Field of the Invention
Aspects of the present invention relate to techniques for use in a wireless communication system. More particularly, aspects of the present invention relate to techniques for channel state information feedback in a wireless communication system.
2. Description of the Related Art
First release 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) does not meet the International Mobile Telecommunications (IMT)-Advanced requirements for 4th Generation (4G) cellular wireless communication systems. Accordingly, a 4G version of LTE is being developed that is referred to as LTE-Advanced (LTE-A). A challenging aspect in the development of LTE-A is improving upon first release 3GPP LTE in the area of average cell throughput and cell-edge user throughput. It has been suggested that DownLink (DL) higher order Multiple Input Multiple Output (MIMO) systems and DL Coordinated MultiPoint (CoMP) transmission may be used in LTE-A to meet the IMT-Advanced DL spectral efficiency requirements. To facilitate these enabling technologies, it has been agreed to only use dedicated antenna ports to support DL transmission demodulation in LTE-A systems.
Since DL transmission is based on Dedicated Reference Signals (DRSs), a channel feedback report would be beneficial in LTE-A systems. In first release 3GPP LTE systems, channel feedback is based on the properties of a transmission scheme. For example, Precoding Matrix Indication (PMI), Channel Quality Indication (CQI), and Rank Indicator (RI) reports together specify a particular transmit scheme at the network side. Even though the feedback is related to the DL wireless channel, information on the transmission scheme is also included. Accordingly, in the DL transmission, the network informs User Equipment (UE) of which transmission scheme it is using through Transmit Pre-coding Matrix Indication (TPMI). However, in LTE-A systems, it will not be necessary for the network to inform the UE about the transmission scheme being used, and thus the UpLink (UL) feedback may be focused on accurately feeding back information on the wireless channel from the UE to the network.
A method referred to as multiple description code has been proposed to achieve better feedback performance for LTE-A systems. The scheme takes advantage of the different realizations of similar channels using independent codebooks. Here, the UE will feedback the precoding matrix indices or channel direction indices of different codebooks at different channel feedback reports. Therefore, if the channel is not changing too much between consecutive channel feedback reports, the multiple feedbacks can be combined to generate a more accurate estimate of the channel. While this scheme will not introduce additional UL overhead, it does require the network to know all the different codebooks.
While this technique may improve the channel feedback quality, it does have drawbacks. For example, this technique may only work in a situation where the channels between different UL feedbacks are highly correlated. If the channels are independent, this scheme actually will impair system performance, since the network will be making decisions on the precoding of the DL transmission based on irrelevant channel feedback reports.
Another technique that is being considered for LTE-A systems relates to Coordinated MultiPoint (CoMP) and, in particular, to the CQI feedback of CoMP. In LTE-A systems, an average cell throughput and a cell-edge user throughput are targeted that are much higher than that of first release 3GPP LTE. CoMP is considered to be one of the more promising techniques to achieve this goal. CoMP transmission has been classified into two categories, namely coordinated scheduling and/or beam-forming, and CoMP joint transmission.
In the class of coordinated scheduling and/or beam-forming, data for a UE is simultaneously transmitted from one cell (also referred to as evolved Node Bs (eNBs) or Base Stations (BSs)) while scheduling decisions are coordinated to control the interference generated in a set of coordinated cells. In other words, the data intended for a particular UE is not shared while some information related to the channels and the controls are shared among different cells. In this class of operations, the signals received from other cells are treated as inter-cell interference and are avoided in the spatial, frequency or time domain.
On the other hand, in the class of joint processing/transmission, data for a UE is simultaneously transmitted from multiple transmission points to improve the received signal quality and/or actively cancel interference for other UEs. In this case, data intended for a particular UE is shared among different cells and is jointly processed at these cells. As a result of this joint processing, the received signals at the intended UE will be coherently or non-coherently added together. In this class of operations, the signals received from other cells are treated as useful signals which may contribute to a higher received Signal-to-Noise Ratio (SNR) at the UE. Within this mode of operation, two classes of transmission schemes are identified, namely CoMP Single User (SU)-MIMO and CoMP Multi-User (MU)-MIMO.
To enable the joint transmission of the data for a UE, the serving cells should have knowledge of the channels between the severing cells and the intended UE, which is similar to that of transmission in first release 3GPP LTE. However, additional information regarding the joint channel is needed for enabling CoMP joint transmission. An example of CoMP joint transmission from two cells (or eNBs) is described below with reference to FIG. 1.
FIG. 1 illustrates CoMP joint transmission from two cells according to the related art.
Referring to FIG. 1, two cells (eNB1 and eNB2) are performing CoMP joint transmission to UE1. UE1 receives respective signals from both eNB1 and eNB2. Instead of treating one of the received signals as interference, both the signals are intended for UE1 and are superposed with each other over the air. The received signal at UE1 may be represented by:Y1=H11w1X1+H21w2X1+N1  (1)where NTi denotes the number of transmit antennas at eNB i, NR denotes the number of receive antennas at UE1, H11 denotes the channel gain from eNB1 to UE1, H21 denotes the channel gain from eNB2 to UE1, Y1 denotes the NR×1 vector of received signal at UE1, X1 denotes the intended message for UE1, wi denotes the NTi×v precoding vector of transmitted signal at eNB i, N1 denotes the NR×1 Additive White Gaussian Noise (AWGN) vector, and V denotes the number of transmission layers of signal X1.
In order to enable the CoMP joint processing operation, the network determines which set of cells will be transmitting to a particular CoMP UE and the corresponding channel information. An example of this process will be described below with reference to FIG. 2.
FIG. 2 illustrates sets of cells used in a CoMP joint processing operation according to the related art.
Referring to FIG. 2, for a given CoMP UE, the network configures a set of cells (referred to as a COMP measurement set 210). The COMP measurement set 210 is substantially the same as a measurement set in first release 3GPP LTE systems. The CoMP UE measures the channels between it and the cells of the COMP measurement set 210. Based on the measurement, the CoMP UE reports channel information (i.e., channel coefficients, precoding matrix indices, channel quality indices, etc.) to the network. The CoMP measurement set 210 may be configured by the network with the assistance of the CoMP UEs. After obtaining the channel information, the network determines a set of cells (referred to as active CoMP transmission points 220) that send a CoMP Physical DL Shared Channel (PDSCH) to the CoMP UE and perform the CoMP joint processing.
The configuration of the CoMP measurement set should be semi-statistic and UE-specific. This is crucial for the UL overhead since the UL channel quality reporting of CoMP UE is directly tied to the CoMP measurement set. Due to how demodulation of CoMP PDSCH has been determined to be performed, the network will have complete freedom in deciding the active CoMP set. In this sense, the CQI feedback of the CoMP becomes difficult because, when computing CQI, certain transmission modes have to be assumed.
Based on an analysis of CQI reporting mechanisms for CoMP, it has been suggested that to improve spectral efficiency of the DL CoMP joint processing, accurate link adaptation is needed. However, since the CoMP transmission points are completely transparent to the UEs. UEs have to figure out what the possible CoMP transmission modes are and compute the CQIs accordingly. CQI feedback mechanisms are mainly classified into two classes, namely individual feedback and joint feedback.
In individual feedback, the CQI values are computed assuming that each individual cell within the CoMP measurement set is transmitting while treating the signals from all the other cells as interference (even the other cells within the measurement set). This class is similar to first release 3GPP LTE CQI feedback.
In joint feedback, the CQI values are computed assuming that all the cells within in the CoMP measurement set are jointly transmitting to the UE. This serves as the best CQI value.
A scheme has been proposed to use mixed CQI feedback and have the UE feeds back some CQI values within the individual feedback together with the joint feedback. However, the main drawback of this approach is that it is extremely difficult to combine the individual feedback with joint feedback to obtain the CQI values for other transmission modes since CQI values are Modulation Coding Schemes (MCS) related to the Signal to Interference-plus-Noise Ratio (SINR) values of the received signals.
Therefore, a need exists for new techniques for channel state information feedback in a wireless communication system.